This invention relates generally to filter changers for fluids comprising apparatus to replace a clogged filter with a fresh filter within a fluid flow passage. More particularly, it relates to screen changers useful in the polymer industry for the filtration of heated, pressurized polymer in liquid form as it flows from an extruder to any of various types of polymer forming equipment.
Typically, in the polymer application two or more filters are mounted in a line on a carrier assembly adapted to locate the filters successively in a filter aperture that communicates between upstream and downstream polymer flow passages. The filter carrier assembly slides within a filter channel that extends transversely of the polymer passages through the filter aperture to the exterior of the filter changer, whereby clogged filters are made accessible for cleaning or replacement with fresh filters. In the following description, a filter is described as "on line" when it is fully aligned in the filter aperture, and a "transition" is the time interval of a displacement of the filter carrier assembly that replaces one filter by the next in the on line position.
A principal object of the invention is to provide improved means for sealing against the escape of polymer through the filter channel, typically under high pressure applied by the extruder, during both the periods when filters are on line and the transition intervals.
A further object is to provide improved means which, in addition to providing the foregoing sealing functions, are adapted for use in continuous flow changers that purge each filter of air and prefill it with polymer before it is aligned in the filter aperture.
Typical screen changers comprise a body forming upstream and downstream flow passages with a filter aperture communicating between the passages, and also forming a filter channel transverse to the passages and extending through the filter aperture to the exterior of the body. A filter carrier assembly is slidably mounted in the filter channel for translational or rotational movement.
Simple slide plate screen changers are illustrated by U.S. Pat. No. 3,675,934 to Heston and U.S. Pat. No. 5,439,589 to Whitman. These patents disclose annular seals fitted in the body and surrounding the filter aperture, bearing against the filter carrier assembly or plate.
The polymer pressure and/or mechanical pressure is applied to the seal upstream of the filter aperture, causing the seal to bear upon the filter carrier assembly or the filter then located in the filter aperture, thereby containing the polymer and preventing its flow into the filter channel. In this type of changer the sealing surface area is generally of small size and located close to the filter aperture, but sufficient to comply with, and remain in sealing contact with, small irregularities in the mating surfaces of the filter carrier, the filter channel and the flow passages.
In these simple changers, during the on line condition the pressure within the filter channel surrounding the sealing surface is substantially atmospheric because large clearances, typically one-half to two inches, exist between the filter carrier assembly and the filter channel. A large pressure differential is therefore effective across the seal and causes it to bear upon the on line filter. However, during a transition polymer readily escapes into the filter channel at a rate that is often sufficiently high to vent the polymer pressure to atmospheric pressure, disrupting the forming process. Therefore, the transition must be a short interval to reduce the loss of polymer and the resulting process disruption.
Where the filter changer has filters spaced so that a filter moving into alignment with the filter aperture becomes partially exposed to the upstream passage pressure before the active area of the filter moving out of alignment ceases to be exposed to such pressure, the flow of fluid from the upstream passage to the downstream passage is continuous during the transition. This provides an opportunity for purging air from the on-coming filter and prefilling it with polymer before it becomes fully aligned in the filter aperture. In U.S. Pat. No. 4,025,434 to Mladota a purge conduit is located so that it is isolated from the flow passages while a filter is on line, but provides a path for the purging of air from an on-coming filter during the transition movement as it enters the filter aperture, allowing it to be prefilled with polymer. In U.S. Pat. No. 4,588,502 to Zibell et al, alternative means for evacuating air from the on-coming filter are provided in the form of a suction pump.
In these continuous flow changers, excessive polymer leakage must be suppressed throughout the transition while the on-coming filter is being purged of air and the exiting filter remains exposed to the upstream polymer pressure. Taking into account the dimensions of the filters as measured in the direction of the transition movement, this requires sealing means that are effective over an area considerably greater in extent than that employed in simple slide plate changers. This broad sealing region must extend the width of a filter element away from the flow passage. Experience has shown that compliant contact seals are difficult to construct with sealing regions large enough to contain the polymer adequately. Consequently, the structural parts of screen changers of this type must be fabricated with extreme precision and bulky dimensions, and at considerable cost. The operating conditions also tend to be sensitive to changes in pressure and temperature of operation, and substantial maintenance costs are typical. In many designs surface irregularities in the sliding plate and the filter channel can only be accommodated by the elastic resilience of the structures.
One technique for providing a broad sealing region without contact between opposing surfaces of the filter channel and filter carrier assembly employs the chilling of a plug of polymer. An example is disclosed in U.S. Pat. No. 3,471,017 to Kalman. The use of chilling apparatus results in complexity of structure and unreliability in the event of pressure or temperature excursions of the polymer melt.
A principal object of this invention is to provide sealing means, particularly for continuous flow filter changers, that do not require the structural bulk, precision of manufacture and complexity of the foregoing types of changers, and do not have the same sensitivity to temperatures, viscosities and pressures, with the associated requirements for precise adjustment and maintenance in use.